In order to achieve a high level of image quality which is as high as that achievable by silver salt photography, with the use of an ink jet recording, it is necessary to reduce the ink jet recording head in ink droplet size, to a degree that it is difficult to see the individual dots formed by the ink droplets from the ink jet recording head. Thus, various ink jet recording heads have been devised, which are roughly 5 pl (pico liter: 10−12 liter) in ink droplet size, 40-50 μm in dot size, and 600×1200-1200×1200 dpi in resolution. Some of them have been put to practical use.
However, in order to satisfy users who want to further reduce an ink jet recording head in the graininess of the halftone areas and highlight areas of a photographic color image which the ink jet recording head forms, the ink jet recording head must be further reduced in ink droplet size; more specifically, the ink jet recording head must be enabled to jet ink droplets which are roughly 2 pl in volume. Obviously, reducing an ink jet recording head in ink droplet size is effective to satisfy the abovementioned user need. However, it unnecessarily reduces the speed at which an ink jet recording head prints a color image which does not need to be printed in high resolution. In other words, it is undesirable from the standpoint of high speed printing.
Thus, various ink jet recording heads structured so that they can be switched in ink droplet size, for example, between roughly 5 pl and roughly 2 pl, have been proposed. When these ink jet recording heads are used for forming a color image, such as an accounting table or a graph, which does not need to be printed in high resolution (in relative terms), they can be set to a relatively large ink droplet size, for example, 5 pl, to achieve a higher image formation speed. However, when they are used for printing a highly precise photographic color image, such as a digital photographic color image, they can be set to a relatively small ink droplet size, for example, 2 pl, to print a high quality image.
Described below are some of the well-known ink jet recording heads which have multiple sets of nozzles, which are different in ink droplet volume.
For example, the specification of U.S. Pat. No. 6,137,502 discloses an ink jet recording head in which ink ejecting openings for ejecting ink droplets of a large size, and ink ejecting openings for ejecting ink droplets of a small size, are arranged so that they are alternately positioned in a zigzag pattern.
The specification of U.S. Pat. No. 6,030,065 discloses an ink jet recording head which has first and second sets of nozzles, which are different in nozzle diameter. In the case of this ink jet recording head, each nozzle is changeable in the diameter of the dot it forms on recording medium, within a preset range, by changing it in the diameter of the ink droplet it jets. The dot diameter ranges of the first and second sets of nozzles partially overlap. Referring to FIG. 9, the specification of this U.S. Pat. No. 6,030,065 also discloses an ink jet recording head, which has sections 10Y, 10M, 10C, and 10K. Each section has a set of small diameter nozzles 12 aligned in the secondary scan direction, and a set of large diameter nozzles 14 aligned in the secondary scan direction; two columnar sets of nozzles in each section are parallel to each other. Further, in terms of the primary scan direction, the four sections are positioned so that the large diameter nozzle sets and small diameter nozzle sets are alternately positioned.
The specification of U.S. Patent Application Publication No. 2003/0214551 discloses an ink jet recording head having multiple ink supply passages for supplying multiple ink ejecting nozzles with ink. This ink jet recording head has a first set of nozzles for ejecting large liquid droplets, a second set of nozzles for ejecting smaller liquid droplets compared to the liquid droplets jetted by the first set of nozzles, and multiple (two) ink supply passages. The first and second sets of nozzles, and the set of liquid supply openings, are aligned in a preset direction, with the two sets of nozzles positioned between the two liquid supply passages.
Further, the specification of U.S. patent Application Publication No. 2004/0021731 discloses an ink jet recording head having multiple (two) first nozzle sets made up of first nozzles which jet liquid droplets of a large size, and a second nozzle set made up of second nozzles which jet liquid droplets smaller than the liquid droplets jetted by the first nozzles. The second nozzle set is positioned between the two first nozzle sets.
The inventors of the present invention made an ink jet recording head in which the volume of each ink droplet jetted by a small diameter nozzle 12 was 2 pl, and the volume of each ink droplet jetted by a large diameter nozzle 14 was 5 pl. The results of the recording tests conducted using this ink jet recording head are as follows. When this ink jet recording head was used to print solid images of a secondary color by ejecting small droplets of magenta and cyan inks, it was likely to yield images which were conspicuously nonuniform. This result seems to be traceable to the following causes. One of the causes is that the smaller the size of a primary ink droplet, the smaller the difference in size between a primary ink droplet, and a secondary ink droplet (satellite ink droplet). The satellite ink droplets are the microscopic ink droplets which are generated as a primary ink droplet is jetted, and are substantially smaller in size than the primary ink droplet. Another cause is that the smaller the size of a primary ink droplet, the greater the number of the resultant satellite ink droplets.
For the purpose of solving the above described problem, it is effective to increase the pass number, that is, the number of times an ink jet head is moved in the primary scan direction across a given area of recording medium to form an image on the recording medium. However, this method directly increases the length of time it takes for an ink jet recording head to complete an image, being therefore undesirable from the standpoint of forming a high quality image at a high speed.
Thus, the inventors of the present invention further studied the mechanism responsible for the formation of a nonuniform image, from a new standpoint, that is, from the standpoint of color (ink) combination for generating a secondary color. As a result, the inventors of the present invention discovered an innovative solution to the above described problem. That is, the inventors discovered that the problem can be solved by changing an ink jet recording head in the arrangement of the nozzles for ejecting ink droplets of the small size.